Fractionation column control



OVERHEAD MAKE BOTTOMS MAKE m m L U M U m A J. T. CABBAGE FRACTIONATION COLUMN CONTROL Filed Oct. 11, 1960 FIG.

S R V E m m M 2 E MB M W T F m m m Y B W R E E l mm D V 0 C B O 4 3 3 6 4 w R 8 3 o m 6 RA 3 FL U M U 4 3 5 mc 3 S R N a Aug. 21, 1962 FEED 3,049,886 FRACTIONATIUN COLUMN CONTROL John T. Cabbage, Bartiesville, @ltla, assignor to Phillips Petroleum Company, a corporation or Delaware Filed Oct. 11, 1960, Ser. No. 61,979 8 Claims. (Cl. 62-21) This invention relates to an automatic operation of a fractionation system. In one of its aspects, the invention relates to a novel method and apparatus arrangement for maintaining a preset overhead accumulator pressure. In another aspect, the invention relates to a control system for maintaining a predetermined constant pressure on a fractionation column. In another aspect, the invention relates to a split range pressure control system which controls a portion of the overhead materials being condensed. In another aspect, the invention relates to the operation of a valved bypass loop and a valved condenser loop in parallel so that one valve is throttling while the other valve is fully open for optimum operation. In another aspect, the invention relates to the control of a proportion of an overhead material being condensed, resulting in controlled accumulator pressure. In a still further aspect, the invention relates to an automatic system for selecting a proper valve for throttling to maintain control with a minimum pressure drop through the control system. In yet another aspect, the invention relates to the control of a cooler bypass valve responsive to the accumulator pressure. In a still further aspect, the invention relates to a method and apparatus of controlling the amount of overhead passed to a cooler responsive to the pressure in the accumulator.

Generally, fractionation comprises a series of boilings and condensations which are performed to separate a feed stream into two or more product streams by means of the ditferenc in vapor pressure or boling points of the constituents being separated. At each step, the vapor leaving the boiling liquid contains more of the material with the lower boiling point than does the remaining liquid. Furthermore, as this vapor is condensed, the liquid that condenses first is richer in the material with the highest boiling point. A fractionation column normally comprises a series of bubble trays which are placed one above the other. These trays are designed so that vapors from a lower tray pass through the liquid in the next tray above. This action condenses a portion of the heavier materials in the vapors, and at the same time vaporizes a portion of the lighter liquid on the tray. In this manner, each tray acts as a reboiler for one distillation unit and as the condenser for the preceding unit. The liquid level on each tray is maintained by a weir placed at one edge thereof. When the liquid level on a tray increases above the weir, the liquid overflows to the next tray therebeneath. The pressure on each tray is maintained for a given throughput by the depth that the bubble cap is submerged below the surface of the liquid and by the friction of the vapors through the bubble caps. The temperature at each tray is maintained by the composition of material on the tray because each tray is at its boiling point, and the equilibrium temperature of the liquid at its boiling point is dependent upon the pressure. Thus, by controlling the pressure at one point in the column, the design of the column will regulate the pressure throughout the column.

In many fractionation operations, a very narrow boiling range cut or fraction is removed overhead from a fractionating column and is passed into an overhead yield and 3,M,886 Patented Aug. 21, 1952 reflux accumulator vessel. Usually, it is desired to completely condense all of this overhead stream and to not yield any gas make from the accumulator. With this type of operation, the accumulator pressure cannot be controlled by a control valve on the gas make. Consequently, the accumulator pressure varies widely, as affected by the condensing unit operation, which in turn is aifected by changes in weather, load, and the like.

I have discovered that the accumulator pressure can be maintained at a desired predetermined pressure through the utilization of a split range pressure control system which controls the proportion of overhead materials being condensed, resulting in controlled accumulator pressure, by regulating control valves in the condenser loop and in the bypass loop so that one of the valves is completely opened while the other valve is being throttled.

Therefore, it is an object of the present invention to provide an improved control system for fractionation columns. Another object of this invention is to provide a control system to maintain a predetermined constant pressure in an accumulator. Another object of this invention is to control the accumulator pressure with less pressure drop which can be resolved into greater capacity with smaller and less expensive equipment.

Other aspects, objects, and the several advantages of the invention are apparent from a study of this disclosure, the drawing, and the appended claims.

According to this invention, there are provided a method and apparatus for the control of the accumulator pressure in a fractionation system through the utilization of a split range pressure control system which controls the proportion of overhead materials being condensed, resulting in controlled accumulator pressure, by regulating control valves in the condenser loop and the bypass loop, so that one of the valves is completely opened While the other valve is being throttled. Further, ac cording to this invention there is provided a pressure control system which automatically selects the proper valve for throttling which will maintain control with minimum pressure drop through the control system.

In the drawings, FIGURE 1 is a schematic View of a fractionation column control system of this invention. FIGURE 2 is a schematic view of a valve control system which is suitable for use in the present invention.

Referring now to FIGURE 1, there is shown a fractionation column 1 which is supplied with an input feedstream by line 2 which enters an intermediate section of column 1. This feedstrearn is maintained at a predetermined rate by means of flow controller 3, which adjusts valve 4 in line 2 in response to the pressure differential across orifice 5 in line 2 upstream from valve 4. Heat is supplied to column 1 by steam coil 6 which is located near the bottom of column 1. Steam is supplied to coil 6 at a constant rate through line 7, which constant rate is maintained by flow rate controller 8, which adjusts valve 9 in line 7 in response to the pressure differential across orifice 1 1, which is disposed in line 7 upstream from valve 9. The spent steam from coil 6 is removed through outlet line 12.

A vapor stream is removed from the top of column 1 through line 13, which passes through valve 14, cooler 15, and valve 16 into reflux accumulator 17. By bypass line 18, having valve 19 disposed therein, bypasses cooler 15 and valve 16. The condensed vapor in accumulator 17 is removed therefrom through line 21. A portion of condensed vapor in line 21 is passed through pump 22 and line 23 back into the upper portion of column 1.

3 A constant rate of flow is maintained through line 23 by flow rate controller 24 which adjusts valve 25 in line 23 in response to the pressure differential across orifice 26, disposed in line 23 upstream from valve 25. Internal reflux controller 27 varies the setpoint on flow rate controller 24- in response to the temperature of the vapors in line 13 and the temperature of the reflux fluid in line 23. The remainder of the condensed vapor in line 21 passes through overhead product line 28, having a pump 29 disposed therein, and through valve 31. The rate of flow through overhead product line 28 is maintained at a value proportional to the liquid level of liquid in accumulator 17 by means of liquid level controller 32, which adjusts valve 31 in line 28 in response to the liquid level in accumulator 17. Split range pressure recorder controller 33 controls valves 16 and 19 responsive to the pressure in accumulator 17, so that one of the valves is completely opened while the other valve is being throtled, resulting in a controlled accumulator pressure. This split range pressure recorder controller 33 is conventional and operates valves 16 and 19 as follows. Valve 16 ranges from fully closed at a first, low pressure signal from split range pressure recorder controller 33 to fully opened at a second, intermediate pressure signal from split range pressure recorder controller 33, and then remains fully opened at any higher pressure signal. Valve 19 remains fully opened from said first, low pressure signal to said second, intermediate pressure signal and ranges from fully opened at said second, intermediate pressure signal to fully closed at a third, high pressure signal from the split range pressure recorder controller 33. That is, at said first, low pressure signal from split range pressure recorder controller 33, valve 16 will be fully closed and valve 19 will be fully opened; from said first, low pressure signal from split range pressure recorder controller 33 to said second, intermediate pressure signal from split range pressure recorder controller 33, valve 16 ranges from fully closed to fully opened while valve 13 remains fully opened; from said second, intermediate pressure signal from the split range pressure recorder controller 33 to the third, high pressure signal from the split range pressure recorder controller, valve 19 ranges from fully opened to fully closed while valve 16 remains fully opened.

The bottom product stream from fractionator 1 is removed through line 41 having a pump 42 and a valve 43 located therein. The rate of withdrawal of the bottom product stream is regulated by liquid level controller 44 which adjusts valve 43 in line 41 responsive to the liquid level in the bottom of fractionator 1. The rate of withdrawal of the vapor stream from fractionator 1 through line 13 is regulated by a pressure recorder controller 45 which adjusts valve 14 in line 13 responsive to the pressure in the top of fractionator 1.

Referring now to FIGURE 2, a control system, which is suitable for use in the present invention, is shown in greater detail. Split range pressure recorder controller 33 can be a conventionally commercially available instrument which operates to regulate an output air pressure signal in response to a pressure under measurement. Thus, split range pressure recorder controller 33 is provided with a first conduit 34 which communicates with the interior of accumulator 17 to measure the pressure in the accumulator 17. The pressure transmitted by conduit 34 actuates bellows or equivalent unit which expands and contracts with changes in pressure, efiecting a positioning of the end of lever arm 36. A source of input air at a predetermined pressure is supplied to split range pressure recorder controller 33 through line 37. Lever arm 36 is pivoted on pivot 38, and has a valve head 39 which controls the amount of air pressure to signal line 40. The air pressure in signal line operates valves 16 and 19. Excess air in split range pressure recorder controller 33 is bled oft" through opening 46.

Although the split range pressure recorder controller is shown in FIGURE 2 as utilizing bellows and air presspaasse sure actuated valves, other types of controllers, utilizing spring bias solenoids, motor driver, and the like are also suitable for use in the present invention, the exact details of the split range pressure recorder controller not being an essential part of the invention.

For purposes of illustration and not in limitation of the invention, the following is given as a specific example of the operation of the invention.

A vapor yield from fractionator 1 at 200 p.s.i.g. passes by the way of the condenser loop 13 and the bypass loop 18- into the accumulator 17 which is to be controlled at :2 p.s.i.g. The liquid is subcooled in the condenser and the bypassed vapors are condensed in the accumulator by the subcooled liquid with equilibrium liquid-vapor conditions existing in the accumulator. As it is desired to operate the accumulator at a pressure of 180:2 p.s.i.g., an air pressure signal in signal line 40 of 0 p.s.i.g. is proportioned to 178 psig. accumulator pressure, an air pressure signal in signal line 49 of 15 p.s.i.g. is proportional to 180 p.s.i.g. accumulator pressure; and an air pressure signal in signal line 40 of 30 p.s.i.g. is proportional to 182 p.s.i.g. accumulator pressure. The bypass valve 19 is biased normally open and remains open at air pressure signals in the range of 0 to 15 p.s.i.g. Valve 19 starts to close at an air pressure signal of 15+ p.s.i.g. and is fully closed at an air pressure signal of 30 p.s.i.g. The condenser loop valve 16 is biased normally shut and starts to open at an air pressure signal of 0+ p.s.i.g. and is fully opened at an air pressure signal of 15 p.s.i.g. and remains fully opened for air pressure signals in the range of 15 to 30 p.s.i.g.

Table of Actuation 0f Valves I Fully open. 2 Throttling. 3 Fully closed.

The sizing of the piping, condenser valve, bypass valve, and flow rates is not critical in the operation of the control system of this invention, which is very advantageous. One of the basic advantages of this invention is the control of the system with less pressure drop, which can be resolved into greater capacity with smaller and less expensive equipment. This advantage is gained by the definite relationship between the position of the bypass valve 19 and the condenser valve 16. The bypass valve 19 remains fully opened while the condenser valve 16 operates through its full travel, thus keeping the cooling requirements on condenser 15 at a minimum; and then if control of the accumulator pressure is not established, the condenser valve 16 remains fully opened while the bypass valve 19 throttles or closes. Thus control is established with an absolute minimum of pressure drop.

As noted above, the drawing is merely diagrammatic and is not intended to fully show all component parts of equipment which one skilled in the art will routinely design for the operation. The showing of an element or piece of equipment does not mean that all such or similar pieces of equipment which may be or can be designed by one skilled in the art in possession of this disclosure cannot be utilized and substituted therefor; likewise the omission of an element which one skilled in the art may include in an actual unit does not means that such a piece of equipment is intended to be omitted simply because it does not appear in the drawing. Sufiice to say, the drawing is for illustrative purposes as is the description thereof.

Reasonable variation and modification are possible within the scope of the foregoing disclosure, the drawing and the appended claims to the invention, the essence of which is that in an operation at a preset differential pressure between tower overhead and accumulator wherein valved bypass loop and valved condenser are operated in parallel, the accumulator pressure actuates control of the valves so that one valve is throttling while the other valve is fully opened for optimum control.

I claim:

1. In a fractionating system including a fractionation column, a cooler, a reflux accumulator, a first conduit communicating with an intermediate section of said column to supply a feed stream thereto, a second conduit communicating between the top of said column and said accumulator through said cooler, a first valve in said sec- 0nd conduit between said cooler and said accumulator,

a third conduit communicating between the top of said column and said accumulator bypassing said cooler and said first valve, a second valve located in said third conduit, a fourth conduit communicating with said accumulator to remove condensed liquid from said accumulator, a fifth conduit for passing a portion of the condensed liquid in said fourth conduit to a section of said column near the top of said column to supply reflux thereto, a sixth conduit for removing the remainder of said condensed liquid in said fourth conduit as an overhead product, a third valve located in said sixth conduit, a liquid level controller on said accumulator, means responsive to said liquid level controller to regulate said third valve responsive to the liquid level in said accumulator to maintain a pre-determined liquid level in said accumulator, a split range pressure recorder controller communicating with said accumulator to establish a pneumatic pressure which is a function of the pressure in said accumulator, means responsive to said pneumatic pressure to control said first valve and said second valve responsive to the pressure in said accumulator, said first valve being normally closed and ranging from fully closed at a first, low value of said pneumatic pressure to fully opened at a second, intermediate value of said pneumatic pressure, and remaining fully opened from said second, intermediate value of said pneumatic pressure to a third, high value of said pneumatic pressure, said second valve being normally opened and remaining fully opened from said first, low value to said second, intermediate value, and ranging from fully opened at said second, intermediate value to fully closed at said third, high value, thereby to maintain a predetermined pressure in said accumulator.

2. In a fractionation system, wherein it is desired to completely condense all of the overhead yield from the fractionation column, including a fractionation column, a first conduit communicating with an intermediate section of said column to supply a feed stream tosaid column, a cooler, a reflux accumulator, a second conduit communicating between the upper end of said column and said accumulator through said cooler, at third conduit communicating between said accumulator and a section of said column near the top of said column to supply reflux to said column, a fourth conduit communicating with said accumulator to deliver an overhead product stream from said system, a fifth conduit communicating with the bottom of said column to deliver a bottoms product stream from said column, and means for supplying heat to the lower portion of said column; a control system for said fractionation system comprising, in combination, a first flow rate controller to maintain a predetermined rate of flow through said first conduit, means to control the heat supplied to the lower section of said column to maintain a constant rate of heat addition to said column, a first liquid level controller communicating with the lower section of said column to regulate the flow through said fifth conduit to maintain a predetermined liquid level in said column, a second flow rate controller to maintain a predetermined rate of flow through said third conduit, an internal reflux controller to regulate the set point on said second flow rate controller responsive to the differential between the temperature of the top of said column and the temperature of the liquid in said third conduit, a second liquid level controller communicating with said accumulator to regulate the flow through said fourth conduit to maintain a predetermined liquid level in said accumulator, a first pneumatically operated valve in said second conduit, a sixth conduit communicating between the upper end of said column and said accumulator by-passing said cooler and said first valve, a second pneumatically opera-ted valve in said sixth conduit, valve means located in said second conduit and said sixth conduit adjacent said upper end of said column to control the removal of overhead vapors from said column responsive to the pressure of said upper end of said column, pressure responsive means communicating with said accumulator to establish a pneumatic pressure which is a function of the pressure in said accumulator, means to apply said pneumatic pressure to both said first valve and said second valve, said first valve being normally closed and ranging from fully closed at a first, low value of said pneumatic pressure to fully opened at a second, intermediate value of said penu matic pressure, and remaining fully opened from said second, intermediate value of said penumatic pressure to a third, high value of said pneumatic pressure, said second valve being normally opened and remaining fully opened from said first, a low value to said second, intermediate value, and ranging from fully opened at said second, intermediate value to fully closed at said third, high value, thereby to maintain a first predetermined pressure in said column and to maintain a second predetermined pressure in said accumulator.

3. In a fractionating system including a fractionation column, a cooler, a reflux accumulator, a first conduit communicating with an intermediate section of said column to supply a feed stream thereto, a second conduit communicating between the top of said column and said accumulator through said cooler, a first valve in said second conduit between said cooler and said accumulator, a third conduit communicating between the top of said column and said accumulator bypassing said cooler and said first valve, a second valve located in said third conduit, a fourth conduit communicating with said accumulator to remove condensed liquid from said accumulator, a fifth conduit for passing a portion of the condensed liquid in said founth conduit to a section of said column near the top of said column to supply reflux thereto, a sixth conduit for removing the remainder of said condensed liquid in said fourth conduit as an overhead product, a third valve located in said sixth conduit, a liquid level controller on said accumulator, means responsive to said liquid level controller to regulate said third valve responsive to the liquid level in said accumulator to maintain a predetermined liquid level in said accumulator, the improvement comprising a split range pressure recorded controller communicating with said accumulator to establish a pneumatic pressure which is a function of the pressure in said accumulator, means responsive to said pneumatic pressure to control said first valve and said second valve responsive to the pressure in said accumulator, said first valve being normally closed and ranging from fully closed at a first, low value of said pneumatic pressure to fully opened at a second, intermediate value of said pneumatic pressure, and remaining fully opened from said second, intenmediate value of said pneumatic pressure to a third, high value of said pneumatic pressure, said second valve being normally opened and remaining fully opened from said first, low value to said second, intermediate value, and ranging from fully opened at said second, intermediate value to fully closed at said third, high value, thereby to maintain a predetermined pressure in said accumulator thereby to maintain a predetermined pressure in said accumulator.

4. In a fractionating system wherein it is desired to completely con-dense all of the overhead yield from the fractionation column, apparatus comprising a fractionatron column, a cooler, a reflux accumulator, a first conduit communicating with an intermediate section of said column to supply a feed stream thereto, a second conduit communicating between the top of said column and said accumulator through said cooler, a first valve in said second conduit between said cooler and said accumulator, a third conduit communicating between the top of said column and said accumulator bypassing said cooler and said first valve, a second valve located in said third conduit, a fourth conduit communicating with said accumulator to remove condensed liquid from said accumulator, a fifth conduit for passing a portion of the condensed liquid in said fourth conduit to a section of said column near the top of said column to supply reflux thereto, a sixth conduit for removing the remainder of said condensed liquid in said fourth conduit as an overhead product, a third valve located in said sixth conduit, a liquid level controller on said accumulator, means responsive to said liquid level controller to regulate said third valve responsive to the liquid level in said accumulator to maintain a predetermined liquid level in said accumulator, a split range pressure recorder controller communicating with said accumulator to establish a pneumatic pressure which is a function of the pressure in said accumulator, means responsive to said pneumatic pressure to control said first valve and said second valve responsive to the pressure in said accumulator, said first valve being normally closed and ranging from fully closed to fully opened over a first range of value of said pneumatic pressure and remaining fully opened for any value of said pneumatic pressure higher than said first range, said second valve being normally opened and remaining fully opened over said first range of value of said pneumatic pressure and ranging from fully opened to fully closed over a second range of value of said pneumatic pressure, said second range being the adjacent higher range to said first range, thereby to maintain a predetermined pressure in said accumulator.

5. A method of controlling a fractionation system wherein it is desired to maintain a predetermined pressure in the accumulator while completely condensing all of the vapor feed to the accumulator, comprising injecting a feed stream at a predetermined rate of fiow into a fractionation column, supplying heat at a constant rate of addition to the lower portion of said column, removing the overhead from said column at a rate of flow responsive to the pressure in the top of said column, removing a bottoms product stream from the bottom of said column at a rate of flow responsive to the liquid level in said column to maintain a predetermined liquid level in said column, passing the overhead removal from said column to an accumulator through a cooling system, said cooling system comprising a first branch having a cooler and a first valve therein, and a second branch having a second valve therein, determining the pressure in said accumulator, controlling said first valve and said second valve responsive to the pressure in said accumulator so that said first valve ranges from fully closed at a first, low accumulator pressure to fully opened at a second, intermediate accumulator pressure and remains fully opened for any accumulator pressure higher than said second, intermediate accumulator pressure, and said second valve remains fully opened from said first, low accumulator pressure to said second, intermediate accumulator pressure, and ranges from fully opened at said second, intermediate accumulator pressure to fully closed at a third, high accumulator pressure; removing the condenser liquid from said accumulator, passing a portion of said condensed liquid to a section of said column near the top of said column as reflux, removing the remainder of said condensed liquid as overhead product at a rate of flow responsive to the liquid level in said accumulator to maintain a predetermined liquid level in said accumulator.

6. In a fractionating system, wherein it is desired to maintain a predetermined pressure in the accumulator while condensing all of the vapor feed to the accumulator, including a fractionation column, a cooler, a reflux accumulator, a first conduit communicating with an intermediate section of said column to supply a feed stream thereto, a second conduit communicating between the top of said column and said accumulator through said cooler, a first valve in said second conduit between said cooler and said accumulator, a third conduit communicating between the top of said column and said accumulator bypassing said cooler and said first valve, a second valve located in said third conduit, a fourth conduit communicating with said accumulator to remove condensed liquid from said ac cumulator, a fifth conduit for passing a portion of the condensed liquid in said fourth conduit to a section of said column near the top of said column to supply reflux thereto, a sixth conduit for removing the remainder of said condensed liquid in said founth conduit as an overhead product, a third valve located in said sixth conduit, a method comprising determining the liquid level in said accumulator, regulating said third valve responsive to the liquid level in said accumulator to maintain a predetermnied liquid level in said accumulator, establishing a signal which is a function of the pressure in said accumulator, varying said first valve from fully closed at a first, low value of said signal to fully opened at a second, intermediate value of said signal, maintaining said first valve fully opened at any valve of said signal higher than said second, intermediate value; maintaining said second valve fully opened from said first, low value to said second, intermediate value, and varying said second valve from fully opened at said second, intermediate value to fully closed at a third, high value of said signal, thereby maintaining the pressure in said accumulator substantially constant.

7. In a method of maintaining the pressure in an accumulator substantially constant, in a system including an accumulator, a cooling means, a source of vapor feed, a first conduit means communicating between said source and said accumulator through said cooling means, a first valve in said first conduit means, a second conduit means communicating between said source and said accumulator, a second valve in said second conduit means; the steps comprising establishing a signal which is a function of the pressure in said accumulator, maintaining said second valve fully opened while varying said first valve from fully shut at a first, low value of said signal to fully opened at a second, intermediate value of said signal, and maintaining said first valve fully opened while varying said second valve from fully opened at said second, intermediate value to fully closed at a third, high value of said signal.

8. In a fractionation system including a fractionation column, a cooler, a reflux accumulator, a first means for providing communication between said column and said accumulator through said cooler, and wherein it is desired to completely condense all of the vapor feed to said cooler; a control system to maintain a predetermined pressure in said accumulator comprising, in combination, a first valve in said first means, second means for providing communication between said column and said accumulator bypassing said first means and said first valve, a second valve in said second means, a split range pressure recorder controller, means communicating accumulator pressure to the responsive element of said controller and means under control of said controller to manipulate said first and second valves responsive to the pressure of said accumulator, said first valve being normally closed and ranging from fully closed corresponding to a first, low accumulator pressure to fully opened corresponding to a second, intermediate accumulator pressure, and remaining fully opened for an accumulator pressure higher than said second intermediate accumulator pressure; said second valve being normally opened and remaining fully opened from said first, low accumulator pressure to said second, intermediate accumulator pressure; said second valve being normally opened and remaining fully opened fnom said first, low accumulator pressure to said second, intermediate accumulator pressure, and ranging from fully opened corresponding to said second, intermediate accumulator pressure to fully closed corresponding to a third, high accumulator pressure, thereby controlling the pressure in said accumulator at a predetermined pressure with a minimum pressure drop between said column and said accumulator.

References Cited in the file of this patent UNITED STATES PATENTS Malkoft et a1 Sept. 4, Barton May 14, Clay Apr. 21, Brooke Dec. 15, Micai et a1 May 3, Schenk et al June 6,

UNITED STATES PATENT OFFICE. CERTIFICATE OF CORRECTION August 21 1962 Patent No. 3,04%886 John T. Cabbage ears in the above numbered patd that error app Patent should read as It is hereby certifie nd that the said Letters ent requiring correction a corrected below.

for "recorded" strike out "a"; line 56 I "predetermniec Column 6, line 28 read recorder column 8 lines 21 and 22 for read predetermined column 8, line 27, for "valve" read value Signed and sealed this 4th day of December 1962.

(SEAL) Attest:

DAVID L. LADD Commissioner of Patents ERNEST W. SWIDER Attesting Officer 

